The present disclosure relates generally to switched reluctance motor controls and, more particularly, to a method and apparatus for determining phase current of switched reluctance electric machines.
A conventional switched reluctance motor (SRM) includes a stator having a plurality of pairs of diametrically opposed stator poles and a rotor having a plurality of pairs of diametrically opposed rotor poles. Windings or coils are typically disposed about the stator poles, wherein the windings around any two diametrically opposed stator poles may be connected in series or in parallel to define one motor phase of the multiphase SRM. The windings associated with a motor phase are also referred to as a phase coils. By generating current through the phase coils, magnetic fields are established about the stator poles and a torque is thereby produced that attracts a pair of rotor poles into alignment with the stator poles.
The current in the phase coils is generated in a predetermined sequence in order to produce a constant torque on the rotor. The period during which current is provided to the phase coil (and during which the rotor poles are brought into alignment with the stator poles) is known as the xe2x80x9cactive stagexe2x80x9d or conduction interval of the motor phase. At a certain point, either as the rotor poles become aligned with the stator poles or at some point prior thereto, it becomes desirable to commutate the current in the phase coil to prevent a negative or braking torque from acting on the rotor poles. Once this commutation point is reached, current is no longer generated in the phase coil and the current is allowed to dissipate from the phase coil. The period during which current is allowed to dissipate from the phase coil is known as the xe2x80x9cinactive stagexe2x80x9d of the motor phase.
The general method of controlling/commutating a switched reluctance motor for servo applications is by regulating the current flowing through each phase as a function of motor position. In order to implement such control, active current feedback information from each phase of the motor is used during the control operation. Typically, the current feedback information is obtained through current sensing devices placed within the individual phases of the motor.
One common type of current sensing device used in SRM control circuit applications is a Hall effect sensor configured within each individual phase of the motor. FIG. 1 illustrates an SRM power circuit 10 having four Hall effect sensors 12 configured within each phase (A, B, C and D) of a four-phase SRM drive. The current sensed by each individual Hall effect sensor is converted into an equivalent voltage, which is thereafter amplified by four corresponding amplification circuits (not shown) and fed into four corresponding A/D channels (not shown) for digital control applications.
In precise torque control applications, such as in electric power steering systems or steer-by-wire systems, for example, a very strict part to part variation tolerance of the Hall sensors is desired in order to keep the torque ripple low. In addition, since each motor phase requires a separate Hall sensor, the costs associated with multiple sensors are increased. As such, it is desirable to implement a current sensing method and system for SRM motors that is both cost effective and that also provides a complete measurement of all phase currents present in each of the motor phases.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for determining phase current for each of a plurality of individual phase coils of a switched reluctance machine (SRM). In an exemplary embodiment, the method includes determining the current passing through a first resistive element and a second resistive element in an SRM drive circuit. A conduction mode of the SRM drive circuit is determined, the conduction mode being based upon an ON/OFF state of each of a pair of switching transistors controlling current through each of the plurality of individual phase coils. The phase current through each of the plurality of individual phase coils is then calculated, based upon the conduction mode and the determined current passing through said first and second resistive elements.
In another aspect, an apparatus for determining phase current for each of a plurality of individual phase coils of a switched reluctance machine (SRM) includes an SRM drive circuit including a pair of switching transistors associated with each of the plurality of individual phase coils. A first resistive element is in selective connection with each of the plurality of individual phase coils, and a second resistive element is in selective connection with each of the plurality of individual phase coils. The phase current for each of the plurality of individual phase coils is calculated based upon values of current sensed in the first and second resistive elements and upon an ON/OFF state of each of the pair of switching transistors.